1. Field of the Invention
The present invention relates in general to a method of manufacturing a cathode ray tube and particularly to a novel knocking method.
2. Description of the Prior Art
In the manufacture of cathode ray tubes such as television picture tubes, metal electrodes which form the electron gun within the cathode ray tube body after being assembled in the cathode ray tube have projections such as flash which is produced during the press forming. Uneven surfaces, dust which adheres to the elements during various processes cause unstable charges to be generated when the cathode ray tube is operating. So as to eliminate such disadvantages it has been known in the prior art to conditionally treat the tube elements by applying a high voltage between a high voltage electrode and a low voltage electrode so as to generate discharges by formation of a strong electric field so as to remove the unstable structure and dust such as flash. This is called "knocking" and as illustrated in FIG. 1, the knocking treatment is performed by applying between an anode button 2 and a terminal pin 4 which is at a low voltage, a high voltage source. The anode button 2 is connected to high voltage electrodes within the tube and the terminal pin 4 is connected to low voltage elements mounted within the neck portion of the cathode ray tube 1 as for example, within the electron gun 3.
The electron gun 3 may be formed in various manners and may, for example, have a unipotential type electron gun illustrated in FIG. 2 which has a first grid G1, a second grid G2, a third grid which is a first anode G3, a fourth grid G4 and a fifth grid which is the second anode G5. Each of the grids may be a metal electrode of cup or cylindrical shape. The third grid G3 and the fifth grid G5 are high voltage electrodes and are electrically connected together and to the anode button 2. The other grids G1, G2 and G4 are electrically connected together and to the terminal pin 4 which extends from a stem of the neck portion of the tube.
So as to perform knocking treatment as described in FIG. 1 to the electron gun in such arrangements, the knocking voltage 5 is applied to the button which is connected to the third grid G3 and the fifth grid G5 of the electron gun 3 and the low voltage electrodes G1, G2 and G4. The knocking voltage source 5 may supply either DC voltage or half wave rectified AC voltage. Usually the knocking treatment is performed by applying DC voltage and AC voltage alternately. However, the method of alternately applying the AC voltage and the DC voltage still does not provide sufficient knocking effect. When DC voltage is used in the knocking treatment, constant high voltage HV is supplied between the high voltage electrodes and the low voltage electrodes continuously as shown in the wave form illustrated in FIG. 3. Alternatively, the DC voltage may be repeatedly applied at regular intervals. In knocking treatment using half wave rectified AC voltage, such as illustrated in FIG. 4, the half wave rectified pulses at a frequency of 60 Hz for example is applied between the high and low voltage electrodes.
When using DC voltage for the knocking treatment, the discharge energy is high because the impedance is low and the effective voltage is high. However, discharges will be generated only at portions having large flash or portions where a large electrical field intensity exists as, for example, between the third grid G3 and the fourth grid G4, or between the fourth grid G4 and the fifth grid G5, or between the second grid G2 and the third grid G3. Also, when using DC voltage, the number of discharges will be small and in other words, the so-called discharge inducing power is small and the overall conditioning of the tube is insufficient.
So as to obtain sufficient conditioning effect using DC knocking treatment, the DC voltage may be increased or the time of applying the voltage may be lengthened. For these conditions however, sputtering of metal materials from the electrode is produced and thereby secondary faults may occur due to the adhesion of the sputtered metal to the inner walls of the neck portion of the cathode ray tube body and damages of various elements 6 mounted close to the electron gun within the neck may occur and also short circuit faults may occur. For example, in cathode ray tubes of the Trinitron (registered trademark) type, a convergence means is mounted in the rear portion of the electron gun. The convergence means is usually supplied with a high voltage from the anode button 2 with the voltage being divided by a bleeder resistor. The bleeder resistor is mounted within the narrow neck portion between the electron gun 3 and the inner wall of the tube. If sputtering is produced as described above, the impedance of the bleeder resistor may be decreased or destroyed.
On the other hand, if knocking treatment uses half wave rectified AC voltage, the discharge energy is weak because the impedance is high and the effective voltage is low. Since the impedance is high and the high frequency component is large, the discharge inducing power is strong and discharges will be generated between the electrodes. However, since the discharge energy is weak, sufficient conditioning effect does not occur to the various electrodes.
Thus, even if DC knocking and AC knocking methods are alternately performed sufficient conditioning does not result and the knocking treatment is not successful. Thus, when completed cathode ray tubes of the prior art are assembled and operated in a television receiver for example unstable discharges may occur during the working state.